GMCSF is a glycoprotein with a molecular weight of approximately 22 kD and is encoded by a gene on chromosome 5q31. GMCSF is produced by many cells of the body, including T cells, macrophages, mast cells, endothelial cells, and fibroblasts. Receptors for GMCSF polypeptides are found on both hematopoietic cells and non-hematopoietic cells and on some tumor cell lines. GMCSF receptors can be heterodimers composed of a ligand-binding α-chain and a β-chain that allows for high-affinity binding and signal transduction. The α-chain can uniquely bind GMCSF, but the β-chain also can be utilized by receptors for IL-3 and IL-5. GMCSF can exert its action locally as a potent growth factor, stimulating the proliferation and maturation of primitive hematopoietic progenitor cells (CFU-GEMM), megakaryocytic progenitor cells, as well as erythroid progenitor cells. It also can stimulate functional activities of many neutrophils, monocytes, macrophages, and dendritic cell functions, enhancing their ability to respond to secondary triggering mechanisms.